In a case where a known turbine blade for a generator is manufactured, the contour of a blade root of the turbine blade must match the shape of a Christmas tree-shaped slot that is formed in the outer circumference of a rotor so that the blade root may be mounted into the slot. The blade root is generally bilaterally symmetrical with respect to its own longitudinal direction, is machined such that the blade root gradually becomes narrower towards its tip, in an upside-down Christmas tree shape in which the width of the root increases and decreases, and is formed such that narrow portions where the root width becomes less alternate with wide portions where the root width becomes greater.
In order to machine the side faces of this sort of blade root in a single pass, for example, a cutting tool of what is called a throwaway type is known in which a tool body that is formed into an upside-down Christmas tree shape is provided and cutting tips may be fastened into a plurality of cutting tip seats that are formed in the tool body (refer to Patent Document 1, for example). This type of cutting tool reduces the volume of discarded tools and is economical, because only the cutting tips may be replaced, while the tool body continues to be used.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-254812
However, with the cutting tool that is described in the Patent Document 1, rough cutting of the complex blade root is done by a single tool, so a heavy load bears on the tool, and in particular, the portions of the tool that are narrower in the radial direction are weaker than other portions. Therefore, vibration and chatter noise may occur during the machining. Moreover, in a case where the machining is continued in this sort of unstable state, the tool may break, particularly in the narrower portions, so the machining must be done while limiting the cutting feed of the tool, accordingly efficient machining may not be done. Furthermore, the shapes of the cutting tips must be matched to the contour of the blade root, so ordinary rectangular cutting tips that are commercially available may not be used. Therefore, the cost of manufacturing the cutting tips may be expensive.